Improving url() compatibility with local files.

This patch lets `url()` remove the query string and hash from the URI before checking if the specified file exists on the server. Kirby includes `url::strip_query()` and `url::strip_hash()` but no function that does both, therefor a new regular expression was created.

(I first tried `[?#].*$` but this seemed to work inconsistently on my PHP version.)

Fixes #97.